The present invention relates to high-power microwave oscillators and, more particularly, to such oscillators using negative resistance diodes. A major objective is to provide for linear frequency modulation ("chirping") for weather avoidance radar systems.
As a precaution against weather-related problems, modern aircraft avionics include weather radar systems to detect atmospheric turbulence. In classical pulse Doppler radar systems, a brief single-frequency high-power pulse is directed into an atmospheric region of interest. Atmospheric moisture partially reflects the transmitted pulse's energy back to the aircraft; a series of reflections from features at various ranges are then received and processed by the system. A feature's range and relative speed can be determined by the reflection delay time and frequency shift caused by the Doppler effect. The aircraft speed can then be factored out to determine the true ground speed of each feature. A velocity versus position map can be constructed to illustrate the weather system. This map can be used by the aircraft to avoid severe turbulence.
To provide earlier anticipation of turbulence, longer range weather radar is desired. Since power falls off as the fourth power of the round trip signal distance, there is a continuing need for increased pulse power. Furthermore, for a given range, higher power means that reflections are more easily distinguished from background noise. This makes it possible to determine range and Doppler shift with greater precision.
Most high-power pulse generators use one or more oscillators combined to allow for higher peak power during transmission. IMPATT (impact-ionization and transit time) diodes can be used as high-power oscillators. When configured with hollow-rectangular or coaxial waveguides as external resonant cavities, high power can be achieved within a narrow frequency band. Multiple IMPATT diodes can be combined (with separate or a shared external cavity) with little loss of efficiency to achieve greater power. Where greater spectral purity is required, Gunn-effect diodes can be used instead of IMPATT diodes, although Gunn-effect diodes produce far less power per diode.
For a given maximum peak pulse power, the pulse's total energy increases with increasing pulse duration. To an extent, greater range and precision can be achieved by taking advantage of the greater energy associated with longer pulses. Offsetting these gains in range and Doppler precision are range errors due to transmitter and receiver bandwidth limitations. Real transmitter and receiver systems have finite bandwidths, which make it impossible to generate pulses with zero rise and fall times. It has been established that, for a bandwidth-limited system, use of a rectangular pulse results in a range error that is approximately proportional to the square root of the pulse duration. Thus, for a given peak pulse power, there is a tradeoff between range and precision.
This tradeoff is addressed by a technique known as "pulse compression." Pulse compression uses a long transmitted pulse to achieve high pulse energy; the pulse reflections are compressed to minimize range errors. Thus, greater range and greater precision can be achieved together.
In pulse compression, the frequency of the transmitted pulse is linearly modulated from a low frequency to a high frequency; this linear frequency modulation is commonly referred to a "sweeping" or "chirping." The difference between the highest and the lowest frequencies is termed the "chirp bandwidth." The chirp serves as a signature that also characterizes the pulse reflections. The received reflection sweeps are then passed through a frequency-dependent delay device, such as a surface acoustic wave (SAW) device, for which the delays imposed vary inversely with frequency. Thus the lower leading frequencies of a reflection sweep are delayed more than the higher frequencies so that all frequencies exit the delay device at the same time. As a result the reflected chirp pulse is compressed into a narrow pulse.
The peak energy of a pulse is augmented by compression so that the reflection is more readily distinguished from background noise. This enhanced signal-to-noise ratio improves range and Doppler precision. Furthermore, the contribution to range error proportional to the square root of pulse duration is obviously reduced due to pulse compression.
Pulse compression requires an oscillator frequency range that meets or exceeds the intended sweep frequency range, and frequency control so that a narrow output frequency can be precisely swept across the range. For purposes of weather avoidance radar, a typical pulse compression scheme could be realized with a sweep covering the 9.1-9.5 GHz range allocated to airborne navigational aids such as weather avoidance radar. This range has a fractional bandwidth which is 4.3% of its center frequency.
Sufficient frequency bandwidth and frequency control can be obtained using transistor-based oscillators. Transistors that are normally used in amplifiers can be used in oscillator circuits; in these circuits the transistor's output is fed back to its input so as to provide positive feedback. The frequency of the oscillator output can be set by a coupled lower power oscillator or resonant circuit. A quartz crystal is commonly used for a fixed-frequency output. A device with a controllable impedance can be used for variable frequency control; a varactor (variable reactance) diode is suitable for this purpose.
The output transistors can be silicon or gallium arsenide bipolar devices, or gallium arsenide metal-semiconductor field effect transistors (MESFETs), depending on the frequency of operation. The highest power transistors are relatively inefficient at microwave and millimeter wave frequencies. Other transistors lose less efficiency when operated at these high frequencies, but have lower maximum powers to begin with. None of the individual transistor oscillators provides an output comparable to that available from IMPATT diodes in the microwave frequency range. (Herein, "microwave" includes centimeter and millimeter wavelengths.)
A large total power can be achieved by arranging many transistor oscillators in a two-dimensional array. Such an array of tunable transistors can provide high-power chirps for weather radar. However, the cost and complexity of such arrays are quite high. High-power bipolar transistors can be expensive. Configuring a system so that all the transistors operate coherently is demanding and expensive.
IMPATT diode oscillators have a substantially greater power output than transistor-based oscillators. It would be desirable to sweep these over the entire 9.1-9.5 GHz range. However, the bandwidth of typical IMPATT diode oscillators is an order of magnitude less than the 4.5% bandwidth of this range. Furthermore, IMPATT diode oscillators typically couple an IMPATT diode with an external cavity such as a hollow rectangular waveguide or coaxial waveguide. These waveguides are characterized by a high Q-factor ("quality"), which generally implies high power efficiency, but narrow bandwidth. The frequency of oscillation is dependent on external cavity dimensions. While it is possible to tune such oscillators by disassembling and reassembling the waveguides and related components, it is difficult to vary frequency in a predictable manner "on line."
There have been several attempts to design a narrow-band matching circuit in a microstrip format for negative resistance devices. In some cases, the active device and its matching circuit have been fabricated monolithically on a gallium-arsenide substrate. Such oscillators operated in a pulsed mode provide high peak power in a compact format and at relatively low cost. The tradeoffs are spectral purity and maximum power relative to oscillators using waveguides as external cavities.
Inherently narrow-band external cavity oscillators, such as those using hollow-metal rectangular and coaxial waveguide cavities, can be made broad band by the inclusion of resistive elements. However, these resistive elements have a severe effect on output power. Accordingly, this approach to high-power variable-frequency power transmission has not found many applications.
Accordingly, the objectives of achieving variable frequency and high power in a microwave transmitter are at odds. Higher power can be achieved at a single frequency, and variable frequency can be achieved at lower powers. Very large arrays of variable-frequency low-power oscillators offer a complex and expensive approach to meeting both of the conflicting objectives. What is needed is an economical variable-frequency high-power oscillator. This would allow for high-power broad-band controllable oscillators needed for pulse-compression radar, and thus longer range and more economical avionics weather avoidance systems.